Demonstrates how to create sites in which any user, regardless of accessibility needs, can access every element, even if they have an outdated search engine, slow modem, small device, or other less-than-optimal conditions. Original. (Intermediate)
A good compendium of design / best practices / markup for most common accessibility issues and problems. Pretty good overview for someone who has already read about accessibility (aka why it is needed and how it should work in theoretical sense), but who doesn't code with it in mind on daily basis.